The regulatory scheme meant to assure air quality has long since passed the "low hanging fruit" and have now moved into the "great cost, low impact" zone.
UPDATE: Nice following editorial "Fright in perspective":
Fright is the go-to tool of environmental activism. When pressure groups want to stifle something—oil well drilling, pipeline construction, whatever—they first arouse fear about the activity among local populations. Then they besiege public officials with their coalition of the frightened.Rumors of ghoulies and ghosties have long been used to keep people scared - and under control.
Industrial societies must be able to assess the industrial activity central to their progress with informed judgment. Those too easily frightened lose that ability and too readily foreswear work.
If you don't think so, try telling the "true believers" you are a man-made global warming agnostic.